Method and device for regenerating the loaded detergent in a physical gas scrubber

ABSTRACT

The invention relates to a process for scrubbing medium regeneration in a physical gas scrubbing, in which predominantly substances of a first type are separated from a laden scrubbing medium ( 2, 17 ) by stripping (cold stripping) in a stripping column (enrichment column (A)) and predominantly substances of a second type are separated from a laden scrubbing medium ( 2, 17 ) by subsequent thermal regeneration in a thermal regeneration column (H), and to an apparatus for carrying out the process. The laden scrubbing medium ( 6 ) is, after the cold stripping, withdrawn from the enrichment column (A), preheated (E 1 ), expanded (a) and subsequently subjected to a thermal desorption in a device for carrying out a thermal desorption (thermal desorption device (W)), in which the content of substances of the first type in the laden scrubbing medium is further reduced.

The invention relates to a process for scrubbing medium regeneration ina physical gas scrubbing, in which predominantly substances of a firsttype can be separated from a laden scrubbing medium by stripping (coldstripping) in a stripping column (enrichment column) and predominantlysubstances of a second type can be separated from a laden scrubbingmedium by subsequent thermal regeneration in a thermal regenerationcolumn, and an apparatus for carrying out the process.

Physical gas scrubbing operations make use of the property of liquids ofabsorbing gaseous substances and of keeping them in solution, withoutchemically bonding these substances in the process. How well a gas isabsorbed by a liquid is expressed by the solubility coefficients: thebetter the gas dissolves in the liquid, the greater its solubilitycoefficient. The solubility coefficient is temperature dependent andgenerally increases with decreasing temperature.

Subsequent to the gas scrubbing, the laden scrubbing medium isregenerated by removal of the scrubbed-out gas components dissolved inthe scrubbing medium. The regenerated scrubbing medium is normallyreused in the gas scrubbing, while the scrubbed-out gas components areeither disposed of or exploited economically.

Physical scrubbing operations are preferably used for the purificationof crude synthesis gases, which are produced on the industrial scale ingasification plants from coal and/or hydrocarbon feedstocks, for exampleby reforming with steam or by partial oxidation, and which generallycomprise some undesirable constituents, such as water, carbon dioxide(CO₂), hydrogen sulphide (H₂S) and carbonyl sulphide (COS). Theseprocesses are suitable since crude synthesis gases are currentlygenerally produced under high pressure and the effectiveness of physicalscrubbing operations increases to a first approximation linearly withthe operating pressure. Methanol scrubbing is of particular importancefor the purification of crude synthesis gases. It makes use of the factthat the solubility coefficients of H₂S, COS and CO₂ in methanol differstrongly from those of hydrogen (H₂) and carbon monoxide (CO). Sincethese differences increase with falling temperature and accordingly theH₂ and CO losses decrease, the methanol scrubbing medium is for the mostpart introduced with a temperature lying far below 0° C. into ascrubbing column and is brought into intimate contact with the synthesisgas to be purified.

For regeneration, the laden methanol scrubbing medium according to thestate of the art is withdrawn from the scrubbing column and first fed tothe upper zone of an “enrichment column”, which is a stripping column.In the enrichment column, a stripping gas conveyed countercurrentwiseextracts predominantly CO₂ from the methanol scrubbing medium, which iscooled down in the process. The cold produced in this way is at the sametime used to reduce the unavoidable losses in cold of a methanolscrubbing operation.

The methanol scrubbing medium, certainly reduced in its CO₂ content butstill laden with sulphur compounds and residual CO₂, is withdrawn fromthe bottom of the enrichment column, preheated and fed to a thermalregeneration column, where it is separated into a bottom productexhibiting a pure scrubbing medium and a top fraction consistingpredominantly of sulphur compounds.

In order to increase the profitability of such a synthesis gasscrubbing, the top fraction from the thermal regeneration column is fedto a sulphur extraction plant (e.g., a Claus plant) and there convertedto elemental sulphur. The top fraction can accordingly be exploited inthis way but its CO₂ content may not exceed a limiting value. Inparticular then, if the crude synthesis gas to be purified exhibits alow sulphur content, such as is the case, for example, with crudesynthesis gas produced by the gasification of low-sulphur coal thislimiting value can be achieved only at high cost (e.g., by coldstripping with an increased amount of stripping gas) or cannot beachieved at all.

It is accordingly an object of the present invention to provide aprocess of the type mentioned at the start and also a device forcarrying out the process through which the disadvantages of the state ofthe art can be circumvented.

This object is, in terms of the process, achieved according to theinvention by withdrawing the laden scrubbing medium from the enrichmentcolumn after the cold stripping, preheating, warming up, expanding andsubsequently subjecting to a thermal desorption in a device for carryingout a thermal desorption (thermal desorption device), in which thecontent of substances of the first type in the laden scrubbing medium isfurther reduced.

The term “thermal desorption” is in this connection to be understood insuch a way that the expulsion of undesirable substances from thescrubbing medium to be regenerated is carried out at higher temperaturesthan with those of the cold stripping preceding the thermal desorptionand at lower temperatures than with those of the thermal regenerationsucceeding the thermal desorption. Because of the strong temperaturedependence of the solubility coefficients, the separation of thesubstances of the first type from the laden scrubbing medium isachieved, through the use of the thermal desorption according to theinvention, with a smaller amount of stripping gas than is possible bycold stripping alone.

Preferred process variants provide for the preheated laden scrubbingmedium to be subjected, for thermal desorption, to an expansion or astripping (hot stripping) with a stripping gas (hot stripping gas). Anadditional preferred process variant provides for the preheated ladenscrubbing medium to be subjected, for thermal desorption, to anexpansion and the liquid fraction formed in the expansion to be treatedby hot stripping with hot stripping gas.

Another preferred embodiment of the process according to the inventionprovides for the same gas, preferably nitrogen, to be used as strippinggas both for the cold stripping and the hot stripping.

With increasing hot stripping temperature, i.e. with increasingtemperature with which the laden scrubbing medium is fed to a hotstripping, the amount of the substances of the second type alsoincreases, which amount passes over from the laden scrubbing medium intothe hot stripping gas and is carried off with this from the hot stripperas “top gas”. If the substances of the second type for example have aneconomic use, it is desirable to separate these substances as completelyas possible from the scrubbing medium to be regenerated, i.e. to extractsaid substances with the highest possible yield. A preferred embodimentof the process according to the invention accordingly provides for thegas stream (top gas) produced in a hot stripping, consisting of hotstripping gas and the substances separated from the laden scrubbingmedium, to be withdrawn from the top of the hot stripper and to be fedto the enrichment column in its lower part. In the enrichment column,the substances of the second type are scrubbed out from the top gas bythe scrubbing medium to be regenerated, which is conveyedcountercurrentwise, and in this way finally end up in the stream ofsubstances of the second type produced in the thermal regenerationcolumn.

If the thermal desorption according to the invention comprises, asprocessing stage, simply an expansion of the preheated laden scrubbingmedium, the gas phase formed in this connection is composed largely ofsubstances of the first type, so that the gas phase, because of its highpurity, e.g., can have a direct economic use.

As already explained further above, the effectiveness of a physical gasscrubbing increases with falling temperature of the scrubbing mediumused. Accordingly, an embodiment of the process according to theinvention provides for the energy for the preheating of the ladenscrubbing medium withdrawn from the enrichment column to be drawn fromregenerated scrubbing medium, resulting in the regenerated scrubbingmedium being cooled down. In this way, at least a portion of the coldproduced in the cold stripping is used for the purpose of compensatingfor the losses in cold which are unavoidable in practice in a physicalgas scrubbing.

A preferred embodiment of the process according to the inventionprovides for the purification, in the physical gas scrubbing, withmethanol as scrubbing medium, of a crude synthesis gas comprising carbondioxide (CO₂) and sulphur compounds. In this connection, the CO₂represents the first type of substance and the sulphur compoundsrepresent the second type of substance which are separated separatelyfrom the laden methanol scrubbing medium.

The invention furthermore relates to an apparatus for scrubbing mediumregeneration in a physical gas scrubbing, exhibiting a stripping column(enrichment column), for the separation of substances of a first type,and a thermal regeneration column, for the separation at least ofsubstances of a second type, from a laden scrubbing medium.

In terms of the apparatus, the object set is achieved by providing,between the enrichment column and the thermal regeneration column, atleast one heat exchanger for preheating a laden scrubbing mediumwithdrawn from the enrichment column and one device for carrying out athermal desorption (thermal desorption device), it being possible forthe laden scrubbing medium preheated in the heat exchanger to beintroduced into the thermal desorption device for an additionalseparation of substances of the first type.

Preferred embodiments of the apparatus according to the inventionprovide for the thermal desorption device to be implemented as expansionvessel or as stripping column (hot stripper) or as combination of anexpansion vessel and an in-line hot stripper.

Another preferred embodiment of the apparatus according to the inventionprovides for it to be possible to supply the same gas, preferablynitrogen, as stripping gas both to the enrichment column and to a hotstripper.

During the thermal desorption, in addition to substances of the firsttype, those of the second type are also separated from the ladenscrubbing medium in a hot stripper and pass over in the stripping gasused in the hot stripper. In order to separate from one another thesubstances of the first and second types present in the stripping gas,one embodiment of the apparatus according to the invention provides fora hot stripper to be connected to the enrichment column in such a waythat the gas stream (top gas) produced in the hot stripping in the hotstripper from the stripping gas and the substances separated from theladen scrubbing medium can be introduced into the lower part of theenrichment column. Advantageously, the apparatus according to theinvention comprises a pump situated between the enrichment column andthe hot stripper, by means of which the pressure of the laden scrubbingmedium withdrawn from the enrichment column can be boosted. Preferably,the pressure can be boosted by the pump thus far that sufficientpressure is available for the expansion of the laden scrubbing medium inthe hot stripper and the top gas flows back to the enrichment columnwithout additional compression.

If the thermal desorption device comprises simply an expansion vessel,variants of the apparatus according to the invention provide for it tobe possible to make direct economic use of the gas phase formed in theexpansion vessel.

An additionally preferred embodiment of the apparatus according to theinvention provides for the possibility of transferring heat energy inthe heat exchanger from a scrubbing medium stream, fed back to thephysical gas scrubbing, to a laden scrubbing medium withdrawn from theenrichment column.

The apparatus according to the invention is particularly suitable foruse in methanol scrubbing operations, in which CO₂ (first type ofsubstance) and sulphur compounds (second type of substance) are scrubbedout from a crude synthesis gas comprising at least carbon dioxide (CO₂)and sulphur compounds and are separately extracted during theregeneration of the laden methanol scrubbing medium.

The invention makes it possible to isolate substances of a first typefrom a laden scrubbing medium considerably more effectively than ispossible according to the state of the art. Using the same amount ofstripping gas, much less of the substances of the first type pass intothe material stream produced by thermal regeneration, which ispredominantly composed of substances of the second type. The substancesof the second type are accordingly extracted in pure form and canaccordingly be further treated more simply and more economically.

In the following, the invention should be explained in more detail bymeans of an exemplary embodiment represented diagrammatically in theFIGURE. The exemplary embodiment concerns a part of the scrubbing mediumregeneration in a physical gas scrubbing in which a crude synthesis gascontaminated with carbon dioxide (CO₂) and sulphur compounds is scrubbedout in a scrubbing column with cold methanol scrubbing medium. Nitrogenis used as stripping gas both in the enrichment column and in the hotstripping column.

Laden methanol scrubbing medium is introduced into the upper section ofthe enrichment column A via line 1, where a portion of the amount of CO₂present therein is stripped off by means of nitrogen, which isintroduced into the enrichment column A via the lines 2 and 12 asstripping gas. The temperature of the methanol scrubbing mediumdecreases so much down to reaching the tower plate K that it becomesvirtually impossible for stripping of the CO₂ to continue. The ladenmethanol scrubbing medium is accordingly withdrawn from the enrichmentcolumn A via line 3, conveyed to a (not represented) heat exchanger andthere preheated in indirect heat exchange with regenerated methanolscrubbing medium flowing towards the scrubbing column (not represented).The preheated laden methanol scrubbing medium is carried via line 4 backinto the enrichment column A and there subjected to a further stripping,in which both its CO₂ content and its temperature are reduced. Thestripping gas, laden with stripped-off components (predominantly CO₂),is withdrawn from the top of the enrichment column A via line 5, whilecold methanol scrubbing medium, which still comprises residual carbondioxide and the bulk of the sulphur compounds, is conveyed out of thebottom space S1 via line 6.

The pressure of the cold laden methanol scrubbing medium is boosted bymeans of the pump P1, before it is fed via line 7 to the heat exchangerE1. Here it is preheated in indirect heat exchange against regeneratedmethanol scrubbing medium supplied via line 8 and evacuated via line 9.The laden methanol scrubbing medium is heated up so much that, afterbeing withdrawn from the heat exchanger E1 via line 10 and beingexpanded via the throttle element a, it enters the hot stripper W with atemperature which is greater than the temperatures with which themethanol scrubbing medium to be regenerated is introduced into theenrichment column A. Additional CO₂ and sulphur compounds are strippedoff from the laden methanol scrubbing medium in the hot stripper W, towhich nitrogen is conveyed as stripping gas via line 11. The ladenstripping gas is evacuated as top gas via line 12 and introduced intothe lower part of the enrichment column A. The sulphur compounds presentin the top gas are scrubbed out in the enrichment column A by ladenmethanol scrubbing medium and thus again end up back in the bottom spaceS1.

Methanol scrubbing medium, poor in CO₂ but still laden with sulphurcompounds, is withdrawn from the hot stripper W via line 13 using thepump P2 and introduced into the heat exchanger E2 via line 14, where itis preheated in indirect heat exchange against regenerated methanolscrubbing medium from the bottom space S2 of the thermal regenerationcolumn H supplied via line 15 and evacuated via line 16. The ladenmethanol scrubbing medium is introduced via line 17 into the upper partof the thermal regeneration column H heated via the reboiler E3 and thesulphur compounds are largely separated therein from the laden methanolscrubbing medium and fed in highly concentrated form via line 18 to aClaus plant (not represented) for economic use.

1. A process for regeneration of scrubbing medium from a physical gasscrubbing step, in which predominantly substances of a first type areseparated from a laden scrubbing medium by cold stripping in a strippingenrichment column and predominantly substances of a second type areseparated from a laden scrubbing medium by subsequent thermalregeneration in a thermal regeneration column, characterized in that theladen scrubbing medium, after the cold stripping, is withdrawn from theenrichment column, preheated, expanded and subsequently subjected to athermal desorption so as to reduce the content of substances of thefirst type in the laden scrubbing medium, said thermal desorption beingconducted at a higher temperature than said cold stripping and a lowertemperature than said thermal regeneration.
 2. A process according toclaim 1, wherein the preheated laden scrubbing medium is subjected, forthermal desorption, to an expansion.
 3. A process according to claim 1,wherein the preheated laden scrubbing medium is subjected, for thermaldesorption, to a stripping with hot stripping gas or to an expansion anda subsequent hot stripping.
 4. A process according to claim 3, whereinstripping gas both for the cold stripping and for the hot stripping isthe same gas.
 5. A process according to claim 4, wherein a top gasstream produced in the hot stripping, comprising hot stripping gas andthe substances separated from the laden scrubbing medium, is withdrawnfrom the top of a hot stripper zone and fed to a lower part of theenrichment column.
 6. A process according to claim 1, wherein, in thephysical gas scrubbing step, a crude synthesis gas comprising carbondioxide (CO₂) and sulphur compounds is purified with methanol asscrubbing medium.
 7. A process according to claim 5, characterized inthat, in the physical gas scrubbing step, a crude synthesis gascomprising carbon dioxide (CO₂) and sulphur compounds is purified withmethanol as scrubbing medium.
 8. Apparatus for regeneration of scrubbingmedium from a physical gas scrubbing, comprising a stripping enrichmentcolumn for the separation of substances of a first type from a ladenscrubbing medium and a thermal regeneration column for the separation ofsubstances of a second type from a laden scrubbing medium, characterizedin that at least one heat exchanger for preheating a laden scrubbingmedium withdrawn from the enrichment column and one device for carryingout a thermal desorption (thermal desorption device) are providedbetween the enrichment column and the thermal regeneration column, itbeing possible for the laden scrubbing medium preheated in the heatexchanger to be introduced into the thermal desorption device for anadditional separation of substances of the first type.
 9. Apparatusaccording to claim 8, characterized in that the thermal desorptiondevice is implemented as expansion vessel.
 10. Apparatus according toclaim 8, characterized in that the thermal desorption device isimplemented as stripping column (hot stripper) or as combination of anexpansion vessel and an in-line hot stripper.
 11. Apparatus according toclaim 8, characterized in that the hot stripper is connected to theenrichment column in such a way that the gas stream (top gas) producedin the hot stripping in the hot stripper from the stripping gas and thesubstances separated from the laden scrubbing medium can be introducedinto the lower part of the enrichment column.
 12. Apparatus according toclaim 8, characterized in that, in the heat exchanger, heat energy canbe transferred from a scrubbing medium stream, fed back to the physicalgas scrubbing, to a laden scrubbing medium withdrawn from the enrichmentcolumn.